Stable-surface alloy steel resistant to acids



Patented May 7, 1929. 0

UNITED STA TIES PATENT, OFFICE- RALPH P. DE VRIES, OF MENANDS, AND HENRYA. DE FRIES, 0F ALBANY, NEW YORK,

ASSIGNORS TO LUDL'UM STEEL COMPANY, OF WATERVLIET, NEW YORK, A COR-ronn'rron or new JERSEY.

STABLE-SURFACE ALLOY STEEL RESISTANT TO ACIDS.

No Drawing. 7

This invention relates to alloys of steel and more particularly to suchalloys which are commonly designated as stable surface acid resistantsteels.

The main objects of our invention are to provide an alloy steel of newcomposition which will be resistant to sulphuric acid in all percentagesof dilution, whether the acid be used in the cold or in the hotcondition, and which will further be resistant to mixtures of sulphuricand nitric and to certain dilutions of hydrochloric acid.

The invention is of particular utility for steel tanks, valves, andfittings of various kinds which are used where they come into contactwith acids such as are employed in commercial or manufacturingoperations, and finds particular application where steel is required forstrength and is at the same time exposed to sulphuric acid diluted toany degree.

The alloy steel of our invention contains nickel from 14%130 30%,chromium from 6% to 20%, silicon from 1% to 3%, tantalum from 1% to 3%,carbon from as low as possible content up to 1%. Manganese may bepresent in amounts usually found in alloy steels, up to .7 5%, and theusual amounts of metalloids, such as phosphorus and sulphur may bepresent. The carbon content varies in all cases with the amount ofnickel, and their approximate relative proportions are expressed by theequation :20 1.6+ .06 1, where m is the carbon content in per cent and ythe nickel content in per cent.

l/Ve do not wish to be held exactly to the equation just given, but asnear as we can determine it sets forth the limiting condition betweenthe carbon and nickel content.

For best results, we have found that the silicon and the tantalum mustalways be used in approximately e a1 quantities; that is, if silicon isused in th amount of 1- it is desired to have talitalum in a nearlysimilar amount. i

As already stated, alloy steels made within this range of compositionare particularly resistant to t e action of sulphuric acid, whether theacid is used cold or hot.

The following is a gpod example of a pre- Application filed November26,1926. Serial No. 151,013.

ferred analysis which we wish to make in this range Carbon 0.8mManganese 0.50 Nickel 18.00 Chromium 10.00 Silicon 2.00 Tantalum 2.00

The attack which occurs is most' pronounced during the first 24 hours.Thereupon the material becomes extremely passive and corrosion proceedsat a uniform, but almost negligible rate. The protecting film formedduring the first exposure of approximately 2 k hours is of such a naturethat it prevents, to all ntents and purposes, further corrosion. It 1simportant to note that the protecting film does not alternately breakdown and rebuild with consequent loss of the acid resistant alloy steel.

The steel after attaining passivity is in a state of equilibrium withregard to acid attack, even if the degree of acid dilution is changedfrequently; that is, the tank containing acid might be washed out withwater and the passivity of the steel previously set up through acidattack would not be broken down.

The corrosion tests against sulphuric acid attack have shown that theloss in weight in all dilute sulphuric acid solutions below 20% is aboutequal. When the strength of the acid is increased from 20% to40%, the attack on the alloy steel diminishes and after the strength of the acidsolution is increased from 50%to 100%, the attack is again slightlyincreased, being equal in the latter case to the losses in the diluteacids below 20%. The acid attack, no matter what the dilution orcondition of the acid, is'so small in amount as to be practicallynegligible for most commercial purposes of which we have any knowledge.

The corrosion resistance of our alloy steel against mixed acids, forinstance, as sulphuric and nitric, is approximately as good as againstsulphuric acid of different solutions.

In contra-distinction to most materials of which we have any knowledgeas being good casts with a good surface and can be cast into any form orshape desired, such castings can be annealed by heating from 1400 to1600 F. and slowly cooled and when so annealed can be easily machined.Rolled and forged products can be annealed with similar\ results.

The physical properties of this material are good. The alloy steelhaving the tensile strength of about 85,000 pounds in the annealedcondition and good impact shock resistance. These properties can bemodified by heat treatment.

a Essentially, the steel is not, however, a

steel which can attain tool steel hardness. By air cooling after heatingto 2100, it will be possible to get a hardness of about 400 Brinnellwhich will be hard enough for many parts that must be hardened to resistwear; while at the same time resisting the attack of acids and acidfumes.

The alloy steel of our invention need not be cleaned of the hot scale ofrolling or hammering or casting to makeit resistant to the acid attack.The scale that is formed is'very smooth and adherent. For many purposesit will be desirable to remove this scale and this may be done by theordinary processes of machining and grinding. The clean metallic surfacethus obtained will be equally resistant to acid attack.

established.

The thermal coefiicient of expansion of this allo steel, is not againbroken down.

We find that the additions of tantalum are peculiarly effective in thealloy steel of our invention to maintain the passivity of the steelagainst acid attack once this has been We claim:

1.' As a new product, an alloy steel resistant to sulphuric and mixedacids, comprising nickel from 14% to 30%, chromium from 6% to 20%,silicon from 1% lum from 1% to 3%, and the balance iron.

2. An alloy steel resistant to sulphuric and mixed acids, comprisingnickel from 14% to 30%, chromium from. 6% to 20%, silicon from 1% to 3%,tantalum from 1% to 3%, carbon in all percentages of the nickel rangebeing substantially in accordance with the equation a=1.6.06y, where a:is the carbon and y the nickel content.

3. An alloy steel resistant to sulfuric acid, comprising nickel from 14%to 30%, chromium from 6% to 20%, silicon from 1% to 3%,,tantalum from 1%to 3%, carbon from 0% to 1%, and the balance iron, the lesser amounts ofcarbon being used with the greater amounts of nickel.

' RALPH P. DE VRIES.

HENRY A. DE FRIES.

carbon from 0% to 1% to-'3%, tanta-

